Stephen Colbert won't take over CBS's "The Late Show" until Tuesday, but he has already amassed millions of YouTube views for skits like "Colbeard" and his segment for a Michigan public access cable TV program called "Only in Monroe."

It's a tactic that's become integral to the success and hype in late-night TV, where the younger generation of hosts has found big audiences for the segments they post on the web and platforms such as Snapchat. For advertises, however, the increasingly viral nature of late-night is both a blessing and a curse.

"If you ask people if they watch Jimmy Fallon, the answer is generally yes, but that doesn't mean they are watching it at 11:30 on NBC," said Michael Law, exec VP and managing director-video investment, Denstu Aegis. "Late night is being consumed across all screens, and social is playing a big role in the shows' popularity."

That's a challenge for advertisers and the networks alike.

CBS sells pre-roll ads for YouTube clips of its late-night shows, and ABC will start selling ads before YouTube clips of "Jimmy Kimmel Live" this season -- in both cases giving a significant revenue cut to YouTube that wouldn't be necessary if these views took place on the networks' own platforms. NBC has yet to reach a deal with YouTube, and its videos run without any advertising at all.

Marketers are often interested but also like to be able to plan ahead, which is not a strong suit of web video. "While the broadcast audience is pretty consistent, what clips are going to become viral are hard to predict and capitalize on in the same way," said Tim Hill, senior VP, group partner-integrated investment, UM.

The scenario in some ways mirrors the rest of TV, where viewing often occurs away from the traditional screen and its 30-second commercials. But it seems exacerbated in the late-night arena.

Jimmy Fallon's "Lip Sync Battle" with Emma Stone, for example has been viewed 59 million times on YouTube over the past year, while Jimmy Kimmel's "Mean Tweets" skit with President Obama has been viewed about 30 million times in the past five months. It's gravy for the program, but those views aren't benefiting its TV advertisers.

Earlier in the year, NBC Universal CEO Steve Burke said that about 70% of the views of "The Tonight Show" occur online and "the majority of those views are un-monetized."

Networks instead have considered digital viewing on platforms like YouTube largely as marketing vehicles to attract new audiences. The problem is, those viewers aren't then necessarily finding their way to the linear airing.

While the introduction of new talent, who have loyal digital followings, is attracting younger audiences, late-night ratings haven't seen much growth from last year, and in some instances have shed audiences year-over-year.

Total viewers for NBC's "The Tonight Show with Jimmy Fallon" dropped 10%, while "Late Night with Seth Meyers" fell 11%; ABC's "Jimmy Kimmel Live" is flat; and CBS's "The Late Late Show with James Corden" is down 1% compared to predecessor Craig Ferguson during the same time last year.

Regardless, the daypart was in hot demand during this summer's ad haggle. Both buyers and sellers alike point to late-night as one of the sole areas of strength in an otherwise lackluster upfront.

"Digital isn't doing anything for ratings but if there's a lot of talk around it, advertisers want to be there," said Catherine Warburton, chief investment officer, Assembly.

Ad buyers should gauge late-night "not necessarily based solely on ratings growth, but rather on content creation, social engagement and some of the softer metrics of success," said Jason Kanefsky, exec VP-strategic investments, Havas Media. "It is highly unlikely that the daypart will generate more than marginal ratings gain in the 2015-2016 season, but the combination of Colbert, Fallon and Kimmel will provide more than its fair share of memorable moments from this TV season."

CBS saw some marketers who haven't purchased commercials in its late-night in several years come back during this year's upfront, said Jo Ann Ross, president-network sales, citing beverage marketers as one example.

And Bank of America will break a new campaign across late-night TV next week. The spots will air throughout the week in "The Tonight Show with Jimmy Fallon," "The Late Show with Stephen Colbert," "Late Night with Seth Meyers" and "The Late Late Show with James Corden."

The heightened interest in late-night with Mr. Colbert debuting on CBS lined up perfectly with the planned start of Bank of America's campaign, said Meredith Verdone, senior VP-enterprise, consumer, global wealth and investment management marketing at Bank of America. The humorous tone of the campaign fit nicely with the humor of late-night TV, she added.

Comedy Central, where Mr. Colbert gained fame as a correspondent on "The Daily Show" and host of "The Colbert Report," is being aggressive in trying to build an ad business off late-night buzz in digital platforms. Even before Trevor Noah fills the seat vacated by Jon Stewart on "The Daily Show" at the end of the month, Comedy Central has hired author, humorist and social activist Baratunde Thurston to oversee content creation for Snapchat, Facebook, Twitter, Tumblr, Instagram and YouTube. That content will be original, not show excerpts.

"It's important for our fans who want updates and to be connected all day and all night on whatever platforms they use," said Chris Ficarra, senior VP-integrated marketing and ad sales, Viacom.

Just as important, it gives advertisers an opportunity to reach consumers outside of the linear broadcast, he said.

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